1. Field of the Invention
The invention relates to a partial objective in an illuminating system of a microlithographic projection exposure apparatus; to a REMA objective that has a partial objective of this kind; and to a microlithographic projection exposure apparatus with a partial objective of this kind.
The partial objective comprises a first and a second lens group that are arranged between a aperture plane and an image plane, an image field to be illuminated being situated in the image plane. The components are arranged centered about an optical axis. Pencils of rays, each with a respective chief ray, enter the partial objective through the aperture plane; the chief rays intersect the optical axis in the region of the aperture plane. The axial distance of the intersection points of the chief rays with the optical axis is here at most 10% of the diameter of the aperture diaphragm. The axial displacement of the intersection points depends on the aberrations of pupil imaging introduced by the portions of the system arranged before the partial objective. Pupil imaging denotes here imaging between pupil planes. The outermost chief ray, which passes through the aperture plane at the maximum angle to the optical axis, strikes the edge of the image field in the image plane. The ray bundle whose chief ray runs along the optical axis defines a central ray bundle. The first lens group then comprises those lenses in which the outermost chief ray has, according to absolute value, smaller ray heights at the lens surfaces than the marginal ray beam of the central ray bundle. The second lens group comprises those lenses in which the outermost chief ray has, according to absolute value, greater ray heights at the lens surfaces than the marginal ray of the central ray bundle. A lens of the second lens group has an aspheric lens surface here.
2. Technical Field
A microlithographic projection exposure device is known from DD 292 727, and has a partial objective of the category concerned between a fly""s eye condensor and a structure-carrying mask. A projection objective follows the structure-carrying mask in the beam path, and images the structure-carrying mask, diffraction limited, onto a photosensitive substrate. The first lens group of the partial objective corresponds to the collimator in DD 292 727. The second lens group corresponds to a field lens consisting of only one lens. The field lens has an aspheric correction surface here, in order to affect the angular distribution of the chief rays in the image plane of the partial objective such that the image plane of the projection objective is illuminated nearly telecentrically. The aberrations of pupil imaging between the aperture plane of the partial objective and the aperture plane of the projection objective are reduced by the aspheric correcting surface. The possibilities of correction of pupil imaging with the arrangement of DD 292 727 are limited, since the field lens consists of only a single lens with positive refractive power. Moreover, the embodiment has only an image-side numerical aperture of 0.04 and a maximum field height of 71.75 mm.
So-called REMA objectives have become known from German Patent Document DE 195 48 805 (U.S. Pat. No. 5,982,558) and German Patent Document DE 196 53 983 A1 (U.S. Ser. No. 09/125,621) of the same assignee. REMA objectives are used in microlithographic projection exposure apparatuses directly before the structure-carrying mask (the so-called reticle). REMA objectives image masking devices, so-called REMA (Reticle Masking) blades, with small blur onto the reticle. The REMA blades are usually embodied by adjustable mechanical blades, by means of which the size of the object field of the following REMA objective can be altered. While a REMA objective with purely spherical lenses is shown in the embodiment of U.S. Pat. No. 5,982,558, the use of aspheric lenses for reducing the number of lenses within a REMA objective is proposed in U.S. Ser. No. 09/125,621. The field lens portion of a REMA objective then matches the angular distribution of the chief rays of the REMA objective to the angular distribution of the chief rays of a following projection objective, in order to attain a continuous course of rays between the REMA objective and the projection objective.
European Patent EP 0 811 865 A2 shows a partial objective, which is arranged between a aperture plane and an image plane. Here not the reticle, but a masking device, is arranged in the image plane of the partial objective, and is imaged onto the reticle by a following objective. Therefore, the partial objective has no direct influence on the distribution of the chief ray angle at the interface between the illuminating device and a following projection objective.
Microstructured components with structure sizes below 0.2 xcexcm can be produced with modem projection objectives. In order to attain these high resolutions, the projection objectives are operated at wavelengths of 248 nm, in particular 193 nm or even 157 nm, and have image-side numerical apertures of greater than 0.65. At the same time, the image field diameter is for the most part greater than 20 mm. The requirements on the optical design for such a projection objective are, therefore, considerable. Besides the field imaging of the reticle onto the photosensitive substrate, the so-called wafer, the pupil imaging is also to be corrected. Thus the forward objective portion, arranged between the object plane and the aperture plane, of a projection objective influences the imaging of the entrance pupil onto the aperture plane, while the rearward objective portion, arranged between aperture plane and image plane, influences the imaging of the aperture plane onto the exit pupil. The aberrations of pupil imaging of the projection objective then become apparent in the distribution of the chief ray angles in the object plane of the projection objective.
The object of the invention is to provide partial objectives of the category concerned, which permit influencing the distribution of the chief ray angles in the image plane of the partial objective over wide ranges. In particular, aberrations of pupil imaging that are introduced by the forward objective portion of a following projection objective are to be compensated.
This object is attained with a partial objective having an optical axis for illumination of an image field, in particular in an illuminating device of a microlithographic projection exposure apparatus. The partial objective is arranged between a aperture plane and an image plane. Pencils of rays, each with a chief ray, start from the aperture plane, and the intersection points of the chief rays with the optical axis are situated apart by at most 10% of the diameter of the aperture plane. The partial objective comprises a first lens group and a second lens group, wherein within the first lens group, an outermost chief ray that passes through the aperture plane at a maximum angle to the optical axis, has according to absolute value smaller ray heights at the lens surfaces than an marginal ray that bounds the pencils of rays whose chief ray runs along the optical axis. Within the second lens group, the outermost chief ray has according to absolute value greater ray heights at the lens surfaces than the marginal rays, and wherein the second lens group has a lens with a first aspheric lens surface, wherein the second lens group has at least a first lens with negative refractive power and at least a second lens with positive refractive power. The maximum field height Yimmax within an image field is at least 40 mm, and the image-side numerical aperture is at least 0.15. The chief rays within the image field have field height Yim and chief ray angles PF between the surface normals of the image plane (IM) and the respective chief rays. The distribution of the chief ray angles PF over the field heights Yim is given by a pupil function PF(Yim), which consists of a linear and a non-linear contribution PF(Yim)=c1Yim+PFNL(Yim), with c1 corresponding to the slope of the pupil function at the field height Yim=0 mm, and the non-linear contribution being at least +15 mrad for the maximum positive field height Yimmax.
The distribution PF of the chief ray angles over the field heights Yim in the image plane of the partial objective according to the invention, the so-called pupil function, can be represented as a polynomial series development with odd powers. The polynomial series is:                               P          ⁢                      xe2x80x83                    ⁢                      F            ⁡                          (                              Y                im                            )                                      =                              ∑            n                    ⁢                                                    c                n                            ·                              Y                im                n                                      ⁢                          xe2x80x83                        ⁢                          (                                                n                  =                  1                                ,                3                ,                5                ,                7                ,                                  9                  ⁢                                      xe2x80x83                                    ⁢                  …                                            ⁢                              xe2x80x83                            )                                                          (        1        )            
The chief ray angles PF, which are determined between the surface normals of the image plane and the respective chief rays, are defined to be negative in the clockwise direction. On grounds of symmetry, the pupil function for a system centered around the optical axis has no contributions with even powers. Were the illuminating system to have no aberrations of pupil imaging in the image plane of the partial objective according to the invention, there would then result for each field height Yim the same axial position of the exit pupil, and thus a homocentric exit pupil. Since in homocentric exit pupils all chief rays intersect the optical axis at one point, there exists exclusively a linear connection between the field height Yim and the tangent of the angle for each chief ray. With very small chief ray angles, as will be the case in what follows, the tangent of the chief ray angle can be directly approximated by the angle. The pupil function for a homocentric pupil has only a linear contribution c1xc2x7Yim, where the coefficient c1 corresponds to the slope of the pupil function at Yim=0 mm. However, due to the aberrations of pupil imaging, there results a different axial position of the exit pupil for each field height. The field-dependent position of the exit pupil is described by the non-linear contribution:                               P          ⁢                      xe2x80x83                    ⁢                                    F                              N                ⁢                                  xe2x80x83                                ⁢                L                                      ⁡                          (                              Y                im                            )                                      =                              ∑            n                    ⁢                                                    c                n                            ·                              Y                im                n                                      ⁢                          xe2x80x83                        ⁢                          (                                                n                  =                  3                                ,                5                ,                7                ,                                  9                  ⁢                                      xe2x80x83                                    ⁢                  …                                            ⁢                              xe2x80x83                            )                                                          (        2        )            
of the pupil function. The contributions of higher order correspond here to the spherical angular aberration of the pupil imaging, and thus to the spherical aberration expressed as angular aberration. An optical system with positive refractive power is usually spherically undercorrected without special correction measures, so that the non-linear contribution PFNL of the pupil function is negative for a positive field height. In contrast to this, the pupil function of the partial objective according to the invention has a non-linear contribution PFNL which is clearly positive for positive field heights. The non-linear contribution
PFNL(Yimmax) to the chief ray angle is at least +15 mrad for the maximum positive field height Yimmax. The partial objective thus introduces a strong overcorrection of the spherical aberration of pupil imaging. This is particularly advantageous because the following projection objective can thereby be spherically undercorrected, and correction means in the projection objective can thereby be saved. It is always more favorable to install these correction means in the illuminating system, since the quality requirements on optical elements in a projection objective are clearly higher than those on optical elements in the illuminating system. The partial objective according to the invention now attains this overcorrection in an image field which has a diameter of at least 80 mm, and whose image-side numerical aperture is at least 0.15. The image-side numerical aperture denotes here the numerical aperture in the image plane, which is possible due to the maximum aperture diaphragm diameter of the partial objective. The entendue (or Helmholtz-LaGrange invariant), which is defined in this case as the product of the image field diameter and the image-side numerical aperture, is at least 12 mm. The overcorrection of the spherical aberration of pupil imaging can be attained when the second lens group of the partial objective according to the invention consists of at least two lenses, a first lens having a negative refractive power and a second lens having a positive refractive power.
It is advantageous for the correction of spherical aberration of pupil imaging when the first lens of negative refractive power has a lens surface concave to the image plane, so that the radius of curvature of this surface is positive. It is then favorable if the ratio of radius of curvature to lens diameter of the concave lens surface is smaller than 1.0, preferably smaller than 0.8. This ratio is bounded below by the value 0.5, which results for a hemisphere. Due to the strongly curved concave lens surface, large angles of incidence of the chief rays of the image points remote from the axis result on this lens surface, and thereby a large contribution results to the overcorrection of the spherical aberration of pupil imaging.
The first lens of negative refractive power is preferably designed as a meniscus. In meniscus lenses, the vertex radii of the front and back surfaces have the same sign.
The first lens with negative refractive power is then to be arranged as close as possible to the image plane. It is advantageous if, up to plane-parallel surfaces such as filters or closure plates, which can also be provided with correction surfaces having random surface profiles, no further optical elements are arranged in the beam path between the first lens and the image plane.
It is advantageous for the correction of field imaging, and thus of the imaging of the pencils of rays in the image plane, when the concave lens surface of the first lens is a surface nearly concentric with the image plane. In this case, the rays of the central ray bundle pencil strike the concave lens surface with small angles of incidence. This occurs when the ratio of the distance of the image plane from the vertex of the concave lens surface to the absolute value of the radius of curvature of the concave lens surface has a value between 0.7 and 1.3. While the rays of the central ray bundle pass nearly unrefracted through the lens surface which is nearly concentric with the image plane, large angles of incidence result for the ray pencils of the image points remote from the axis. The surface can thus be used ideally for the correction of the field-dependent image aberrations, while the central ray bundle remains nearly uninfluenced.
If no further lenses with optical refractive power are situated between the first lens, with the lens surface concave to the image plane, and the image plane, it is advantageous if half the radius of curvature of the concave lens surface is clearly greater or smaller than the distance of the vertex of the concave lens surface from the image plane. This requirement on the radius of curvature of the concave lens surface comes into play when the reticle is situated in the image plane and usually reflects a portion of the incident light rays back in to the partial objective. Since each optical surface has a residual reflection, even with an antireflective coating, the light reflected from the mask would be reflected back again toward the mask at the lens surface concave to the image plane. With a nearly telecentric illumination of the mask, a undesired reflection would result if the image plane were situated at the distance of the focal length of the concave lens surface acting as a mirror. The focal length of a concave mirror is given by half the radius of curvature. A undesired reflection can be neglected when the absolute value of the difference of the distance of the concave lens surface from the image plane and the focal length is greater than the absolute value of the focal length multiplied by the factor 0.3.
Minimizing the undesired reflections can also be considered for the other surfaces of the second lens group. So that no undesired reflections arise between the reticle and the lens surfaces of the second lens group, the second lens group is constructed such that the outermost chief ray, which passes through the aperture plane at a maximum angle to the optical axis, has after a reflection at the image plane and a reflection at a lens surface of the second lens group, a ray height in the image plane which is at least 30% of the maximum field height Yimmax. With a constant refractive power of the partial objective, this can be attained by the variation of the curvature of the lens surfaces. The outermost chief ray is therefore made use of for the estimation of the undesired reflections, since its intersection with the image plane marks the 50% point of the undesired reflection light distribution. If however the outermost chief ray were to intersect the image plane in the region of the optical axis, all further chief rays would likewise intersect the image plane in this region, and the doubly reflected pencils of rays would strike it in a narrow region around the optical axis, thus resulting in undesired gohst image.
The first aspheric lens surface is distinguished by a large sag difference of at least 0.2 mm, preferably 0.4 mm, with respect to an envelope sphere. These large asphericities are a further means of correction in order to provide the overcorrection of the spherical aberration of pupil imaging. The sags are defined as distances between the aspheric lens surface and the envelope sphere in the direction of the optical axis. The envelope sphere denotes a spherical surface which has the same vertex as the aspheric surface and which intersects the aspheric lens surface at the edge of the illuminated region of the aspheric lens surface. The illuminated region is bounded by the marginal rays of the ray bundle of the outermost chief ray.
In addition, the design of the partial objective is made more difficult in that the image-side working distance of the partial objective is to be at least 30 mm, and preferably at least 40 mm. The free working space denotes here the distance of the image plane from the vertex of the last optical surface of the partial objective, this distance being reduced by the maximum sag of the last optical surface in the case that a concave surface is concerned for this surface. The free working distance makes possible free access to the image plane, in which the reticle is usually situated. Apparatuses for the positioning and change of the reticle have to be able to intervene in this space.
It is possible with the partial objective according to the invention to overcorrect the spherical aberration of pupil imaging such that the non-linear contribution PFNL(Yimmax) to the chief ray angle amounts to at least +25 mrad for the maximum positive field height Yimmax.
This can be attained in that, inter alia, the second lens group has a second aspheric lens surface.
The maximum sag difference of the second aspheric lens surface from the envelope sphere is to be as much greater than 0.2 mm as possible, preferably greater than 0.4 mm.
It is advantageous if, for the maximum field height Yimmax, the ratio of the non-linear contribution PFNL(Yimmax) to the linear contribution c1xc2x7Yimmax lies in the range of xe2x88x920.5 and xe2x88x922.0. The non-linear portion of the pupil function can then be partially compensated by the linear portion for the maximum field height, so that for the positive field heights nearly equally large maximum and minimum chief ray angles result, and the chief rays for these field heights run on average parallel to the optical axis. The linear portion of the pupil function is set by the paraxial position of the exit pupil.
Apart from influencing the spherical aberration of pupil imaging, the partial objective focuses the incident ray bundle to spot images with minimum diameter in the image plane. For this purpose, the correction of field imaging is required. The maximum spot diameter for all spot images is preferably 2% of the maximum field height Yimmax. For the determination of the spot image and the spot diameter, the ray bundle at full opening of the aperture diaphragm is considered, so that the pencils of rays illuminate the maximum image-side numerical aperture. The spot image is then given by the penetration points of a ray bundle with the image plane. Primarily, the first lens group is available as the correction means; it advantageously consists of a meniscus with positive refractive power and a meniscus with negative refractive power. In addition, it is favorable to provide an aspheric lens surface in the first lens group.
It is favorable for the simultaneous correction of pupil imaging and field imaging if the second lens is a meniscus with positive refractive power.
The second lens group advantageously consists of three through five lenses, in order to correct field imaging, to overcorrect the spherical aberration of pupil imaging, and to ensure a uniform illumination of the image field.
This is in particular possible by the additional use of a biconvex lens in the second lens group.
The partial objective according to the invention can be advantageously used within a REMA objective, where the REMA objective images an object field with a magnification of three through eight times onto an image field. The REMA objective consists of a first partial objective between the object plane and the aperture plane, and the partial objective according to the invention. The two partial objectives have a common optical axis. The magnification of the REMA objective can be adjusted by means of the ratio of the focal lengths of the first and second partial objectives. Chief rays starting from the object plane do not necessarily intersect the aperture plane in one point when the pupil imaging of the first partial objective or of the optical components arranged before the REMA objective introduce aberrations to the pupil imaging.
Since the REMA objective is to image the masking device arranged in the object plane of the REMA objective as sharply as possible onto the image plane in which the reticle is arranged, the spot images of the object points have a minimum diameter in the image plane. The maximum diameter of the spot images is 2% of the maximum field height Yimmax. For the determination of the maximum spot diameter, pencils of rays are used at the maximum aperture diaphragm diameter, which corresponds to the maximum image-side numerical aperture.
The entrance pupil of the REMA objective is advantageously situated at infinity, so that the chief rays of the pencils of rays after the object plane run parallel to the optical axis and thus telecentrically. By means of this measure, the magnification ratio of the REMA objective is independent of a defocusing of the object, in this case the masking device.
Besides the chief rays, which are defined by means of pupil imaging, the energy-weighted average rays in the image plane of the REMA objective are also of importance. The energy-weighted average ray of a ray bundle here represents the ray which results from an averaging over all the rays of the ray bundle under consideration, where each ray has an energetic weighting according to the illumination of the entrance pupil. For a field height Ym, the direction of the corresponding energy-weighted average ray depends on the aberrations of the REMA objective in connection with the illumination of the entrance pupil of the REMA objective. The energy-weighted average rays can be determined, for example, for a complete illumination of the entrance pupil, or for an only partial illumination of the entrance pupil, the illumination being respectively nearly point-symmetrical with respect to the optical axis. The REMA objective is now constructed such that the maximum angular deviation between the energy-weighted average ray and the chief ray for all field heights is smaller than 2 mrad, preferably smaller than 1 mrad. This requirement, together with the requirements on the field imaging and pupil imaging, is attained with a REMA objective which includes eight through twelve lenses with finite focal length, the first partial objective having three through five lenses, and the second partial objective having five through seven lenses. In addition, the use of three through five aspheric lenses is advantageous.
The partial objective according to the invention is advantageously used in a microlithographic projection exposure apparatus, in which a projection objective directly follows the partial objective. The interface between the illumination system and the projection objective thus represents the image plane of the partial objective or the object plane of the projection objective, respectively. The partial objective and the projection objective are centered about a common optical axis. In order to ensure a continuous course of the pencils of rays of the illumination system and the projection objective, the distribution of the chief ray angles of the partial objective has to be matched to the distribution of the chief ray angles of the projection objective at this interface. The deviation of the pupil function of the partial objective from the object-side object pupil function is in this case advantageously smaller than 2 mrad for all field heights within the image field of the partial objective, preferably smaller than 1 mrad. If this condition is fulfilled, the partial objective and the projection objective then form a functional unit with respect to pupil imaging, matched to the partial objective according to the invention, the projection obejctive can then have a distinct undercorrection of the spherical aberration of pupil imaging, since the aberrations of the projection objective can be compensated with the partial objective. This substantially relieves the optical correction of the projection objective.
Correspondingly, it is advantageous to use a REMA objective in a microlithographic projection exposure apparatus, the REMA objective including a partial objective according to the invention.